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What is DC electricity?
Direct current (DC) electricity, in contrast to AC,
ows in one direction in a wire or cable and the
voltage cannot be increased or decreased
using transormers as it can with AC electricity.
For example, a torch battery uses DC electricity
attaching a wire at either end o the battery
allows electricity to ow in one direction through
the wire powering the light. As shown below,
DC ows in one direction.
DC electricity did not develop as the means o
transmitting bulk power supplies rom power
stations because it required special electrical
equipment to allow conversion to higher
voltages, which was expensive and unreliable
in the early days o electricity transmission.
This meant that the electrifcation o whole
countries was delivered quickly and efciently
by adopting AC technology rather than DC.
This largely remains the case today, with AC
providing the most efcient solution, especially
over short distances.
What is AC electricity?
The electricity that supplies most othe worlds homes and
businesses is
alternating current (AC). Power stations
produce AC electricity at high voltages
but, using transormers at substations,
the voltage o the electricity is increased
urther to transmit it through National
Grids network, which operates at
400,000 and 275,000 volts.
High Voltage Direct Currentelectricity the acts
The combination o
AC electricity and
transormers allows
electricity to be
transported across the
electricity system and
minimises the amount
o electricity lost due tothe resistance within
electricity conductors.
Overhead lines and underground cables
transmit the electricity to towns and cities,
where another set o transormers reduces the
voltage so that the electricity can be distributed
to homes and businesses.
In the UK the whole electricity grid system operates
at the same system requency. Frequency is the
speed with which the current alternates and in
the UK this happens 50 times every second or
50Hertz (Hz). This diagram shows how an
alternating current creates a waveorm as it movesbetween the
maximum and minimum positions.
All the electricity produced at power stations that
goes on to supply our televisions, ridges and
mains lighting is AC and has a requency o 50Hz.
The combination o AC electricity and transormers
allows electricity to be transported across the
electricity system and minimises the amount o
electricity lost due to the resistance within
electricity conductors. Power systems worldwide
have been developed on this technology.
www.nationalgrid.com/uk/electricity/majorprojects/
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However, High Voltage Direct Current (HVDC)
transmission is used in some special cases
where it has technical or economic advantages
over AC transmission. For example, HVDC is
used to connect independent power systems
with each other, such as those in Europe with
the UK, or to connect very remote power
stations, such as oshore wind arms, to the
main transmission network.
Connecting independent
electricity systems with
HVDC electricity
A small amount o electricity currently arrives atNational Grids
network through HVDC cables
rom other countries power systems. There are
HVDC links with France and with Northern
Ireland. Others are under construction to
Holland and to the Republic o Ireland, while a
link to Norway is also being considered.
HVDC provides a practical solution to moving
electricity rom one country to another, because
it protects each countrys AC electricity system
rom any requency changes in the other.
The process works by converting AC to DCat a converter station.
The DC electricity is
then transmitted to the other end and on arrival
converted, at another converter station, to the
AC voltage and requency o the receiving
country or network. When converting electricity
at very high voltages, the converter stations
oten need to be very large or example,
the size o a ootball pitch.
Transmitting HVDC electricity
over long distances
When transmitting electricity over extremelylong distances, HVDC
is more efcient than AC.
This is because the amount o electricity lost
during AC transmission is greater than that lost
in HVDC systems over long distances. HVDC
becomes more efcient than AC when the
distances to be covered are in the hundreds o
kilometres. For example, in China, the
Changzhou HVDC project provides a link to
Shanghai some 850km away via HVDC
overhead lines.
HVDC
technology isthereore used
primarily or
connections to
other countries
or between
remote parts o
the network.
Why cant HVDC be
used everywhere?
The UK electricity system is an integrated
network o transmission lines and substations,
all designed to provide sae and reliable
electricity supplies. The distances between
substations are relatively short, in the tens
o kilometres, and using HVDC in these
circumstances would be very expensive and
much less efcient.
HVDC technology is thereore used primarily
or connections to other countries or between
remote parts o the network.
How much does HVDC cost?
The cost o HVDC increases with the amount o
electrical power it is required to transer and the
distance o transmission. This is because o the
increased cost o converter stations and the
extra cables required when more power is
being transerred.
An AC overhead line can transport large
quantities o power. For the highest voltage levels
used in the UK, transer capacities o 6000MWand above can be
achieved at costs in the
hundreds o millions o pounds (depending on
the length o the line). However, an HVDC cable
can transmit only 2000MW. For an HVDC
solution to provide similar capacity o 6000MW,
several cables would be required and the costs
would be measured in billions o pounds.
For urther inormation regarding DC technology
please reer to our more detailed HVDC actsheet.
Jan 2011www.nationalgrid.com/uk/electricity/majorprojects/
